use functional component as a first example in readme (#14819)

README.md

@@ -40,10 +40,8 @@ You can improve it by sending pull requests to [this repository](https://github.
 We have several examples [on the website](https://reactjs.org/). Here is the first one to get you started:
 
 ```jsx
-class HelloMessage extends React.Component {
-  render() {
-    return <div>Hello {this.props.name}</div>;
-  }
+function HelloMessage({ name }) {
+  return <div>Hello {name}</div>;
 }
 
 ReactDOM.render(